Bone in human and other mammal bodies is generally classified into two types, cortical bone, also known as compact bone and trabecular bone, also known as cancellous or spongy bone. Cortical bone is much denser with a porosity ranging between 5% and 10%. Cortical bone is found primary is found in the shaft of long bones and forms the outer shell around cancellous bone at the end of joints and the vertebrae.
In the vertebrae, each bone is generally heart shaped, with spinous, inferior and superior processes joined to the vertebral body via opposing pedicles. To stabilize or fix the spine, implantable medical devices can be employed between adjacent vertebrae, and attached via screws through the pedicles (i.e., via “pedicle screws”) and other osseous structures such as the lamina and facet joints.
The outer bony shell of the pedicles is formed of dense cortical bone, which surrounds spongier cancellous bone. In degenerative conditions such as osteoporosis or following injury, the cancellous bone can weaken and degrade. Cancellous bone can also be displaced through installation of a pedicle screw.
Screws of all sizes can loosen or shift position if placed in osteoporotic or otherwise weak cancellous bone. Pedicle screws are available which seek to avoid loosening or shifting in place through installation with an expandable anchor. In most such designs, the anchor includes deflectable wings at the distal tip which will push outwardly against the bone as the pedicle screw is inserted through the anchor in a fashion similar to screw anchors used in building walls. However, the cylindrical design of such anchors provides relatively minimal interaction between the body of the screw and bone. They also tend to rely primarily on friction to secure the anchor, which can close and lose essential contact as the cancellous bone further degrades.
Another drawback of many existing pedicle screw implants is the lack of adaptability to use with different surgical devices. In particular, pedicle screws of differing diameters, whether monoaxial or polyaxial in design, generally include a screw head for engaging an installation device, or a headless design. In both instances, the installation device must generally be matched in size and shape to the pedicle screw head or other proximal screw surface, requiring a selection of tools to be available for use with different patients.